P040B

What it means

P040B is the generic code for an EGR temperature sensor 'A' circuit range or performance fault. The engine control unit has seen a signal from the exhaust gas recirculation temperature sensor that is implausible, stuck, or does not change as expected when EGR flow is commanded or stopped. The exact test conditions and sensor location vary by model and engine.

Symptoms

• Engine warning light on
• Possible rough idle or hesitation if EGR flow is also faulty
• Slight loss of performance or economy
• Failed emissions test
• Other EGR-related codes stored alongside it

Possible causes

• Faulty or contaminated EGR temperature sensor (sooted, slow to respond or drifting)likely
• Restricted or sticking EGR valve or blocked EGR passages, so temperature does not change as expectedlikely
• Damaged, chafed or corroded wiring, or a poor connector at the sensorpossible
• Faulty EGR cooler or bypass arrangement affecting gas temperature (design depends on engine)possible
• Exhaust leak or fault near the sensor affecting readingsrare
• Engine control unit fault or outdated calibration, only where the manufacturer has documented itrare

How to diagnose

1. Read all stored and pending codes and note the freeze frame data, including engine temperature, load and speed when the fault set.
2. Check for related EGR, air flow or manifold pressure codes and any manufacturer technical bulletins for the model and engine.
3. Visually inspect the sensor wiring and connector for heat damage, chafing, water ingress and corrosion.
4. Check the sensor's live data with the engine cold, then warm, and compare it with a known ambient or coolant temperature reading.
5. Command the EGR valve open and closed with a scan tool if supported and watch whether the sensor temperature responds sensibly.
6. Measure the sensor's resistance or signal voltage against the manufacturer's specification, and check the supply, ground and signal wiring for continuity and shorts.
7. Inspect the EGR valve, pipework and passages for carbon build-up or restriction, and check the EGR cooler for leaks where fitted.

How to fix

• Repair or replace damaged wiring or connectors at the sensor.
• Replace the EGR temperature sensor if its readings are out of specification or slow to respond.
• Clean or replace a sticking EGR valve and clear blocked passages as the manufacturer advises.
• Repair or replace a faulty EGR cooler or bypass component.
• Fix any exhaust leak near the sensor, then clear the code and road test to confirm it does not return.

Can I keep driving?

Nearby codes

• P040AGenericEGR Temperature Sensor A Circuit
• P040CGenericEGR Temperature Sensor A Circuit Low